For the most part any gundam or robot from the gundam series or super robot wars games are available. Though I would suggest that you choose wisely when it comes to certain mechs since the states are fixed based on the point in the story. That means I would avoid trying to have things like the impulse or freedom until much later in the plot where their stats would be decent.

If you would like to have a mech reserved for later use you can let me know and I will give it the stats and note that it will appear later in the plot. Specialty are not available for everyone and most will be using the mass produced weapons of the side they choose to keep things balanced.

Keep in mind that the stats play a role in if your attacks hit or miss. The higher the speed the better chance you have to avoid attacks. The damage is based on the attack vs defense of each mech. The higher the attack is then the defense the more damage done. If the defense is higher little to no damage is taken.

examples
GM Ground Type vs Gespenst T-Link
Speed: 10 vs Speed: 14 the Gespenst T-Link has a slightly better chance to avoid attacks then the GM Ground.

Defense: 9 vs Strength: 15
The GM would take a decent amount of damage form the attacks made against it.

Strength: 10 vs Defense: 17
The GM will not do as much damage with it's attacks.

If you have any other questions feel free to send me a pm.